In a balance system of canceling an rotation moment of a rotation shaft of a horizontal arm generated by a weight of the horizontal arm in an articulated robot of the background art, there are a counterweight system, a system of utilizing an urge force of a tension spring or a compression spring, or a system of balancing by utilizing a force provided by a hydraulic pressure by using a cylinder and a piston, a system of using both thereof or the like.
According to the counterweight system, a weighted object is arranged at a rotation axis symmetric position of a horizontal arm to directly balance, or a balance is taken by installing a counterweight at a vertical axis rear portion and connected to a horizontal arm by using a link. However, the system is provided with drawbacks that an interfering region of a robot is large, also a weight thereof is heavy, an inertia load is large or the like.
Therefore, normally, it is general to provide a balance force by utilizing a spring or a cylinder urged by a pressurized fluid.
That is, there is constructed a direct spring balance structure in which when a weight of a horizontal arm pivoted by constituting a rotating shaft by a shaft of an upper end of a vertical arm pivoted above a turning base, an upper side of the vertical arm is provided with a member moved in parallel therewith while maintaining a vertical direction, and a tension spring urged obliquely between a front end of the member and the horizontal arm is hung to be expanded.
However, according to the direct spring balance system of hanging to expand the tension spring obliquely between the front end of the member provided on the upper side of the vertical arm and moved in parallel therewith while marinating to be vertical and the horizontal arm of the background art, an expanded portion of the member is large, also the weight is large, and therefore, when a range of operating the robot is going to be widened, a total of the apparatus is enlarged, the spring is exposed to outside, which is dangerous. Further, since the tension spring is used, a fatigue rupture is liable to be brought about at a hanger portion by a stress concentration and when the spring is destructed, there is a danger of dropping the arm.
In this way, according to the background art system, the expanded portion of the member is large and also the weight is large. Hence, there is proposed a constitution of installing a spring for a balance at a vicinity of a turning base (refer to, for example, Patent Reference 1 (JP-A-59-167486)).
According to the constitution, an inclination of a horizontal arm is moved to a vicinity of a rotation drive shaft of a vertical arm in parallel therewith by a parallel link mechanism and a balance force is exerted by a tension spring or a compression spring urged at the position.
According to a structure of a balancer utilizing a cylinder urged by a pressurized fluid (air pressure or oil pressure), the spring of the balancer utilizing the spring is replaced by an air pressure cylinder or an oil pressure cylinder, and a pressure adjusting function of the pressurized fluid is added to be installed.
According to the constitution of balancing by the spring by moving the inclination of the horizontal arm to above the turning base in parallel therewith by the parallel link mechanism disclosed in Patent Reference 1, similar to the above-described background art apparatus, the spring is exposed to outside, which is dangerous, the tension spring is used, and therefore, a fatigue rupture is liable to be brought about at a hanger portion and when the spring is destructed, there is a danger of dropping the arm.
Further, in Patent Reference 2 (JP-A-11-262889), with an object of providing a simple balancer mechanism of a horizontal arm bringing a main body of the balancer to inside of the horizontal arm and using a compression spring, there is proposed to constitute a balancer apparatus in which in an industrial articulated robot constituting a parallel link of moving a link arm and a wrist relative to a turning base in parallel therewith and including a motor fixedly provided to the turning base for driving to pivot an upright arm and a motor for driving to pivot the horizontal arm, a cantilever load by a weight of the horizontal arm at an arm shaft is balanced by a lever rotatably supported by a horizontal shaft provided at an upper side of the vicinity of the center of the horizontal arm and projecting an arm in an up and down direction, a lever rod for coupling a shaft fixedly provided onto the link arm and an upper arm end of the lever by a pin, and a pulling device provided at inside of the horizontal arm for coupling one end portion of the horizontal arm through a pin and coupling a loose end portion to a lower arm end of the lever by a pin.    Patent Reference 1: JP-A-59-167486    Patent Reference 2: JP-A-11-262889